1. Field of the Invention
The present invention relates to a solid-state imaging apparatus and a driving method therefor.
2. Description of the Related Art
Solid-state imaging apparatuses include charge-coupled solid-state imaging apparatuses and amplifying solid-state imaging apparatus, respectively represented by CCD and CMOS sensors. An amplifying solid-state imaging apparatus typically has a photoelectric conversion unit, an amplifying transistor, a transfer transistor, and a reset transistor, in a pixel. Signal charges are transferred by the transfer transistor to a floating diffusion (hereinafter, FD) connected to a control electrode of the amplifying transistor, and amplified by the amplifying transistor to be output. Before signal transfer, the FD is reset by the reset transistor.
In general, when a large amount of incident light impinges on the photoelectric conversion unit of a pixel of a solid-state imaging apparatus, signal charges can overflow the photoelectric conversion unit. It is known that when overflowing electric charges leak into the photoelectric conversion units of neighboring pixels, a false signal called blooming is produced and manifests itself as a white smear on a screen. Under a dark condition with no incident light, a dark current produced in a photoelectric conversion unit manifests itself as a white blemish on a screen, and when such a dark current is produced in a large amount to cause corresponding blooming, the area of the white blemish will be larger.
For an amplifying solid-state imaging apparatus, a countermeasure against such blooming which leads to degradation in image quality is an operation of discharging a certain amount of electric charges into an FD through a transfer transistor while signal charges are being accumulated in a photoelectric conversion unit. Examples of this operation in a CMOS sensor are described in a Japan application publication, Japanese Patent Application Laid-Open No. H11-355664 (hereinafter, Patent Document 1), and a Japan application publication, Japanese Patent Application Laid-Open No. 2008-099158 (hereinafter, Patent Document 2). With a technique of Patent Document 1, control signals supplied to gates of a transfer transistor and a reset transistor during accumulation are biased. Accordingly, the transfer transistor and the reset transistors do not become completely off, releasing excess electric charges to a power source. According to Patent Document 2, during a signal charge accumulation period, a pulse having a lower potential than the potential at the time of signal charge transfer is intermittently applied to the gate of a transfer transistor, and during application of the intermittent pulse, electric charges overflowing the photoelectric conversion unit are discharged to the FD through the transfer transistor.
However, while the technique of Patent Document 2 inhibits blooming, the technique has a disadvantage of lowering sensor characteristics as described below. In the technique of Patent Document 2, inhibition of blooming would be substantially perfect if the above-described operation of discharging excess charge is performed regularly, not intermittently, during the signal charge accumulation period. To do so, however, it is necessary to keep the gate of the reset transistor in high-level state and the gate of the transfer transistor at a medium potential Vm for discharge of excess charge. In such a condition, the photoelectric conversion unit side of an element isolation region over which the gate of the reset transistor is positioned gets close to a weakly inverted state, stimulating production of a dark current. Also, the area below the gate of the transfer transistor becomes a weakly inverted state to increase dark current from a gate interface of the transfer transistor. Such dark current flows into the photoelectric conversion unit to degrade sensor characteristics. Thus, to minimize the time period for which such a condition lasts, the operation is performed intermittently. However, when the operation is performed as regularly as possible for prevention of blooming, dark current will increase for the above-described reason.
From another perspective, because the reset and transfer pulses of the intermittent operation are applied to all pixels of the sensor, when the operation is performed regularly, it leads to increase in consumed current and increase in dark current associated with heat generated from the consumed current. From yet another perspective, an upper-limit quantity of saturated charge that can be intrinsically accumulated in a photodiode reduces with a partial charge discharge operation as described above.
From still another perspective, a study by the present inventors also revealed such a disadvantage as follows. Because both the transfer and reset pulses are at low levels during a period in which the pulse of medium potential Vm for application to the gate of the transfer transistor is not applied, the FD is put in a floating state between 0 V and a reset potential. When focusing on a pixel that is in a dark state or a near dark state, the FD of the pixel can also be close to 0 V in the floating state. This is because FD is high in dark current because of its general structure and such dark current can be sometimes accumulated in the FD, and also because when the amount of incident light impinging on pixels neighboring the dark pixel of interest is large, electric charges overflowing the photoelectric conversion units of the neighboring pixels may flow into the FD of the pixel of interest. When the pulse of medium potential Vm is applied with the FD thus being 0 V or near 0 V, the electric charges in the FD are reversely transferred to the photoelectric conversion unit. As described above, the conventional blooming preventing operation has the disadvantage of having a mechanism that increases white blemishes due to dark current from the FD or possibly broadens blooming instead of eliminating it.
An object of the present invention is to provide a solid-state imaging apparatus that is capable of inhibiting blooming and a driving method therefor.